Refrigeration mechanical diagnostic protection and control device

ABSTRACT

In vapor compression refrigeration systems a mechanism and method are provided for protecting a compressor from failures related to lack of superheat, loss of lubricating oil and other system malfunctions. Also provided is a mean of monitoring system conditions and providing service personnel with a quick manner of diagnosing problems.

BACKGROUND OF THE INVENTION

The present invention relates to refrigeration, air conditioning and heat pumps and more particularly to a method and apparatus for monitoring and controlling system pressures, temperatures, and superheat and subcooling.

In the operation of refrigeration and air conditioning systems, the cooling effect is provided by the change in state of the refrigerant from a liquid to a gas in the evaporator of the system. The gaseous refrigerant is compressed by a compressor and is condensed to a liquid state in a condenser before passing through an expansion valve upon being returned to the evaporator.

The failure of a compressor is usually very costly. Most compressor failures can be traced back to one of the following system conditions: “refrigerant floodback”, “flooded starts”, “slugging”, excessively high discharge temperature or loss of lubricating oil.

“Refrigerant floodback” results when liquid refrigerant returns to the compressor during the running cycle. The lubricant oil becomes mixed with refrigerant to the point that it cannot properly lubricate the load bearing surfaces. This situation can usually be prevented by 1) maintaining proper evaporator and compressor superheat, 2) correcting abnormally low load conditions, and/or 3) installing accumulators to stop uncontrolled liquid return.

“Flooded starts” are the result of liquid refrigerant vapor migrating to the crankcase oil during the off cycle. When the compressor starts, the diluted oil cannot properly lubricate the load bearing surfaces causing erratic wear. This situation can be prevented by 1) locating the compressor in a warm ambient location or installing a continuous pump down system, and/or 2) installing a crankcase heater.

“Slugging” is the result of trying to compress liquid refrigerant and/or oil in the compressor cylinders. Slugging is an extreme floodback condition. This situation can be prevented by 1) maintaining proper evaporator and compressor superheat, 2) correcting abnormally low load conditions, 3) installing accumulators to stop uncontrolled liquid return, and/or 4) locating the compressor in warm ambient location or installing a continuous pump down system.

In the case of excessively high discharge temperature the compressor head and cylinders become so hot that the oil loses its ability to lubricate properly. This causes rings, pistons and cylinders to wear resulting in “blow by”, leaking valves and metal debris in the oil. Excessively high discharge temperature can be corrected by 1) correcting abnormally high load conditions, 2) correcting high discharge pressure conditions, 3) providing proper compressor cooling, and/or 4) providing proper compressor cooling.

Loss of oil is the result of insufficient oil in the crankcase to properly lubricate the load bearing surfaces. When there is not enough refrigerant mass flow in the system to return oil to the compressor as fast as it is pumped out, there will be a uniform wearing or scoring of all load bearing surfaces. To protect against loss of oil 1) check oil failure control operation, 2) check system refrigerant charge, and/or 3) correct abnormally low load conditions or short cycling.

Superheat is defined as the temperature of vapor above the boiling point temperature of its liquid at that pressure. It is calculated in vapor compression refrigeration systems by 1) converting the suction side line pressure to a saturated vapor temperature, using a temperature/pressure chart for the specified refrigerant, 2) measuring the suction side line temperature six inches from the inlet to the compressor, and 3) subtracting the pressure to temperature conversion from the suction line temperature. The result is the system superheat.

Subcooling is a measure of the heat being dissipated to the atmosphere at the exterior heat exchange coils. It is calculated in vapor compression refrigeration systems by 1) converting the discharge line pressure to temperature, using a temperature/pressure chart for the specified refrigerant, 2) measuring the line temperature at the liquid line service port, and 3) subtracting the pressure to temperature conversion from the line temperature. The result is the system subcooling temperature.

In the prior art U.S. Pat. No. 4,563,878 describes a method for compressor protection from low superheat conditions by system shutdown, but does not indicate what caused the failure conditions. Investigation would have to begin, usually involving the connection of refrigerant pressure gauges. Each time that such industry standard gauges are connected to service ports on the refrigerant lines refrigerant is allowed to escape to the atmosphere. Repeated connections and disconnections allow a significant enough volume of refrigerant to escape and make up refrigerant must be supplied to maintain a proper system charge. Another cause for concern is introduction of foreign materials from the gauge set into the system.

Other prior art addresses the monitoring of superheat and initiate compressor shutdown. U.S. Pat. No. 5,209,076 details a microprocessor based control device which will shut a compressor down if a low superheat state is entered. This device does not monitor or display other system parameters such as temperature and pressure. Further it does not provide simple system status indicators for the actual condition of superheat.

U.S. Pat. No. 5,209,076 does not include in its functionality the options for energizing external subcooling or fan cycling relays. This device also relies on analog to digital converters for refrigerant pressure sensing, a method with inherent inaccuracies that would not provide the level of accurate control required with a refrigeration system under heavy load conditions.

Other examples of prior art have been found, such as U.S. Pat. No. 4,038,061 and U.S. Pat. No. 4,545,212 that monitor system conditions and initiate some action towards compressor protection. These devices, however, only address one or two dangerous systems conditions and fail to provide and adequate level of compressor protection.

SUMMARY OF THE INVENTION

Accordingly it is the object of the present invention to protect a refrigeration system or air conditioner compressor against adverse operating conditions such as floodback, slugging, excessively high discharge temperature and loss of oil. The present invention will also protect the system against high refrigerant line pressure and low refrigerant line pressure.

This and other objects of the present invention are attained by monitoring and controlling the system pressures (high and low), temperatures (both system and ambient), superheat and subcooling. The present invention uses a microprocessor to sense pressure inputs using pressure transducers (example: 4-20 mili-amp, 0-10 Volt DC, or resistance) for high side and low side pressures. Temperature sensors are connected to the liquid line, the suction line and positioned to sense outdoor ambient temperature. This information is conveyed to the microprocessor. With this information the firmware installed in the microprocessor can calculate the superheat, subcooling, discharge temperature, high side pressure, low side pressure and outdoor temperature. Through the use of control relays the microprocessor can protect the compressor from mechanical failure from the previously mentioned conditions.

The present invention will also prove useful on initial start up of a refrigeration system or air conditioner by checking the system's refrigerant charge and superheat with the manufacturer's specifications for both.

It is also the purpose of this invention to assist a service technician in determining the present state of the refrigeration system and in rapidly determining what fault condition(s) may have occurred. This is done through the use of a bank of LED indicators showing the present superheat temperature status (“OK”, “WARNING” or “FAILURE”), the low pressure condition, high pressure condition, discharge temp condition, and sensor status. This bank of LEDs represents the first line of diagnosis, allowing even moderately skilled individuals to quickly determine the system's condition and/or reasons for failure. In the past this would have entailed the connection of a set of test gauges to service ports on the refrigeration system, allowing some refrigerant to escape to the environment as well as possibly allowing non-condensables to enter the system.

Further detailed values can be displayed on an LCD digital display which can show all data collected and calculated by the microprocessor.

The present invention is designed to work in conjunction with most sensing and metering devices presently used on refrigeration systems. Examples of these devices include capillary tube, thermostatic expansion valves, fixed orifice and electronic expansion valves.

The invention also allows for condenser fan cycling (either vari-speed or simply ON/OFF) to maintain a head pressure range in low ambient conditions. Under high ambient conditions when proper subcooling is difficult to maintain, the invention can energize an auxiliary subcooling device.

In its preferred embodiment the apparatus of the present invention is installed as a stand alone unitary controller but can easily be connected to building automation controls through the built in communication port. Further, this communication port can be configured to energize a telephone access module to alert service personnel of fault conditions.

Also, the present invention can be configured for different refrigerants (example: R-22 or R410-A) through the use of preprogrammed pressure to temperature charts in the firmware.

In addition to system protection from low superheat, the present invention uses the collected data to further increase system efficiency by providing a method of increasing subcooling under heavy load conditions or decreasing subcooling when needed.

An electrical device monitors the mechanical aspects of standard refrigeration and air conditioning systems. The device protects these systems by calculating temperature, pressure, superheat, sub-cooling, ambient temperature and controlling system components. The device provides an easy, graphic representation of system conditions and faults for rapid verification of satisfactory operating parameters. Further detailed system conditions are provided for more extensive examination through use of digital read-outs.

BRIEF DESCRIPTION OF THE DRAWINGS

Novel features and advantages of the present invention in addition to those noted above will become apparent to persons of ordinary skill in the art from reading the following detailed description in conjunction with the accompanying drawings wherein similar reference characters refer to similar parts and in which:

FIG. 1 is a diagrammatic view of a refrigeration system and a protection system, according to the present invention;

FIG. 2 is a functional block diagram of the protection system of the present invention;

FIG. 3 is the main logic flow chart of the software in the microprocessor controller according to the present invention;

FIG. 4 is a logic flow chart of the software in the sensor polling subroutine, according to the present invention;

FIG. 5 is a logic flow chart of the software in the sensor validity check subroutine, according to the present invention;

FIG. 6 is a logic flow chart of the software in the fault logging subroutine, according to the present invention;

FIG. 7 is an illustration of the device display panel with detachable diagnostic tool, according to the present invention; and

FIG. 8 is an illustration of an alternative device display panel with LED and LCD displays, according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring in more particularity to the drawings, FIG. 1 illustrates a diagram of a standard refrigeration system which includes a compressor 26 driven by an electric motor in a conventional manner. The discharge side of the compressor 26 connects with a discharge line 42 which delivers the compressed refrigerant in a gaseous state to a condenser 10 or in some systems multiple condensers. Near the outlet of the compressor 26 a discharge temperature sensor 36 is connected with the discharge line 42. The gaseous refrigerant condenses into a liquid state in the condenser 10. Located at the inlet air side of the condenser 10 is an outdoor temperature sensor 28.

Exiting the condenser 10, the liquid refrigerant travels in the liquid line 44 to a receiver 12 which stores excess refrigerant during low load conditions.

Exiting the receiver 12 the refrigerant travels through the liquid line 46. Located on the supply line 46 near the exit of the receiver 12 are a high pressure transducer 30 and a subcooling temperature sensor 32. The liquid supply line 46 typically travels through a filter-dryer 14, then through a sight glass 16 and a solenoid valve 18 before entering an expansion valve 20 where the liquid refrigerant changes state back to a gas.

Gaseous refrigerant enters an evaporator 22 where heat is exchanged with the building or refrigerated enclosure. Refrigerant vapor leaves the evaporator 22 and travels through a suction line 48 into a suction accumulator 24 and finally back to the compressor 26. Situated on the suction line 48 after the suction accumulator 24 is a low pressure transducer 40. Situated on the suction line 48 after the suction accumulator 24 but before the compressor 26 is a suction temperature sensor 34. Both the low pressure transducer 40 and the suction temperature sensor 34 are located from 6″ to 18″ from the compressor.

FIG. 2 is a functional block diagram of the present invention in which a device 102 includes a conventional microprocessor 104 which receives power from a power supply and isolation circuits 152 connected to the power interface 128 which receives power from external AC supply 154. The device 102 is factory programmed but can be modified in the field with dip switches 134 on a circuit board. A liquid crystal display (LCD) module 162 may be attached to the device 102.

A detachable module 162 includes an LCD 168 for monitoring of system conditions and a key pad 170 for scrolling through the various system readings. The detachable module 162 has a suitable interface 164 with the microprocessor 104, and the keypad 170 has an interface 166 with the microprocessor 104. The detachable module 162 communicates with the microprocessor 104 through the keypad and display control interface 136.

A real time clock 150 connects to the microprocessor 104 through a suitable interface 126. A reset key 172 is provided on the device 102 and permits the device to be reset by a service technician after a system lockout.

A light emitting diode (LED) display bank 160 has a number of LEDs to show system status at all times. These individual LEDs include, but are not limited to, “System OK”, “Low Pressure Failure”, “High Pressure Failure”, “Discharge Temperature Failure”, “Sensor Failure”, “Superheat OK”, “Superheat Warning”, “Superheat Failure”. The self diagnosis LED interface 132 on the microprocessor 104 sends a voltage output to the display bank 160 based on inputs from external sensors and calculations performed in the microprocessor 104.

There are six external sensor inputs: outdoor temperature sensor 106, suction temperature sensor 108, discharge temperature sensor 110, liquid line temperature sensor 112, low pressure sensor 114 and high pressure sensor 116. All the external sensors communicate with the microprocessor 104 through a sensor signal interface 118.

The device 102 is provided with memory circuits that connect with the microprocessor 104 through a memory interface 124. The memory circuits includes random access memory (RAM) 146 having a battery backup 148, programmable read only memory (PROM) 144 and an electrically erasable read only memory (EEPROM) 142.

An equipment cut off and alarm relay control 130 on the microprocessor 104 communicates with a normally open relay 158 which controls power to the compressor 26.

A subcooling relay interface 120 on the microprocessor 104 is a normally open contact that would be closed based on calculations performed in the microprocessor 104. Voltage output would energize a subcooling relay 138 which would operate an external device to maintain proper subcooling.

A fan cycling relay interface 122 on the microprocessor 104 is also a normally open contact that would be closed based on calculations performed in the microprocessor 104. Voltage output would de-energize a fan motor relay 140, shutting off the condenser fan. This will help to maintain a constant refrigerant head pressure during low ambient temperature conditions. The condenser fan would be allowed to resume operation when system conditions demand.

A communications port interface 174 sends information to the communications port 156 so the information from the microprocessor 104 can be shared with existing building control systems.

FIG. 3 is a logic flow chart of the software in the microprocessor controller 104. From a start block 202, block 204 is entered to get readings from all sensors, including refrigerant pressure before the compressor, refrigerant pressure after the compressor, refrigerant discharge temperature, refrigerant suction line temperature, refrigerant temperature at the receiver and ambient temperature. These values are stored for future use.

In block 206 the values returned from block 204 are evaluated for plausibility. If they are zero or outside the possible high limit it is determined that there is a sensor failure. Block 208 evaluates the result of the sensor check performed in block 206. If it is determined that a sensor failure has occurred we proceed to block 234, a fault and logging subroutine, from there to block 242 for a system shut down, and then to block 244 to terminate the logic loop.

If all sensors respond and pass plausibility test in block 208, block 210 is entered and a two minute start up delay is initiated. This delay is to prevent an operator from initiating repeated system starts in rapid succession. If a system shutdown were to occur, a two minute delay may be enough time for minor system difficulties to stabilize enough for a successful startup and to provide protection against compressor short cycling.

After the two minute startup delay, block 212 is entered wherein the compressor start relay is energized causing the compressor to start.

Block 214 polls all sensors as in block 204.

Block 216 tests for sensor validity as in block 206.

Block 218 evaluates the response from block 216. If there is a sensor failure then block 236, a fault and logging subroutine, is entered.

If all sensors respond and pass plausibility in block 218 then block 220 is entered, wherein the discharge side refrigerant temperature is evaluated. If the refrigerant temperature is found to be within acceptable limits, block 222 is entered. Block 222 evaluates the liquid refrigerant pressure, and if the pressure is found to be within acceptable limits then block 224 is entered. Block 224 checks for refrigerant low pressure. If the refrigerant pressure is found to be above the low limit then block 226 is entered. Block 226 evaluates the calculated value for superheat to determine if it is within preset limits. If superheat is acceptable then block 228 is entered to determine if external system subcooling is required. If external subcooling is required block 230 is entered and an external subcooling relay is energized. After block 230 or if subcooling was not required, block 232 is entered where “system OK” LED is energized and all other system warning LEDs are turned off.

Block 234 is then entered where a 0.5 second logic delay is entered. This delay is built in to reduce the number of sensor polling events to a reasonable number. Polling 20 or 30 times a second is not required. After the logic delay in block 234, block 214 is reentered, all sensors are polled and the logic loop continues.

In the first logical pass through blocks 220, 222, 224 and 226, the dectections are as follows.

If block 220 detects a value higher than the compressor manufacturer specified maximum for discharge temperature then a system fault has occurred.

If block 222 detects a value higher than the compressor manufacturer specified maximum for refrigerant pressure then a system fault has occurred.

If block 224 detects a value lower than the compressor manufacturer specified minimum for refrigerant pressure then a system fault has occurred.

If block 226 detects a value lower than the compressor manufacturer specified minimum for superheat (typically 3 degrees) then a system fault has occurred.

If a system fault is determined in blocks 220, 222, 224 or 226 then a fault and logging subroutine 236 is entered.

After returning from block 236, block 238 determines if the system has faulted more than three times for the same reason. If three sequential faults have occurred then a system shut down 242 is initialed. If less than three sequential faults have occurred then block 240 is entered initiating a 90 second system delay before attempting a compressor re-start.

FIG. 4 is a logic flow chart of the software in the sensor polling subroutine 204. Entering in block 302, block 304 communicates with the discharge temperature sensor 36. This value is stored in memory.

Block 306 communicates with the high pressure transducer 30. This value is stored in memory.

Block 308 communicates with the low pressure transducer 40. This value is stored in memory.

Block 310 communicates with the suction line temperature sensor 34. This value is stored in memory.

Block 312 communicates with the subcooling temperature sensor 32. This value is stored in memory.

Block 314 communicates with the outdoor temperature sensor 28. This value is stored in memory.

Block 316 consults the temperature/pressure table for the selected refrigerant (example: R-22) stored in the programmable read only memory (PROM) 160 and converts the value returned by the low pressure transducer to the low side saturated temperature value.

Block 318 calculates system superheat. Superheat is the low side saturated temperature minus the actual suction line temperature. The result is expressed in degrees Fahrenheit and stored in memory.

Block 320 evaluates the calculated superheat value. If superheat is greater than twenty degrees it is determined to be acceptable and block 328 is entered wherein the “Superheat OK” LED is energized. If block 320 determines that superheat is less than twenty degrees then block 322 is entered.

Block 322 evaluates the calculated superheat value. If superheat is greater than three degrees then block 326 is entered wherein the “Superheat Warning” LED is energized. If block 322 determines that superheat is less than three degrees then block 324 is entered.

Block 324 energizes the “Superheat Failure” LED. After one of the three superheat indicator LEDs are energized block 330 is entered.

Block 330 consults the temperature/pressure table for the selected refrigerant (example: R-22) stored in the programmable read only memory (PROM) 160 and converts the value returned by the high pressure transducer to the high side saturated temperature value.

Block 332 calculates system subcooling. Subcooling is the high side saturated temperature minus the actual liquid line temperature. The result is expressed in degrees Fahrenheit and stored in memory.

Block 334 returns logic flow to the parent program.

FIG. 5 is a logic flow chart of the software in the sensor validity check subroutine 206. Entering in block 402, block 404 checks the value returned by the discharge temperature sensor 144 to see if it is greater than zero.

Block 406 checks the value returned by the high pressure transducer 30 to see if it is greater than zero.

Block 408 checks the value returned by the low pressure transducer 40 to see if it is greater than zero.

Block 410 checks the value returned by the suction line temperature sensor 34 to see if it is greater than zero.

Block 412 checks the value returned by the subcooling temperature sensor 32 to see if it is greater than zero.

Block 414 checks the value returned by the outdoor temperature sensor 28 to see if it is greater than zero.

If any of the above values are determined to be zero then the variable SENSORS_RESPOND_OK=NO is determined in block 418. Otherwise if all the values are determined to be greater than zero then the variable SENSORS_RESPOND_OK=YES is determined in block 416.

Block 420 returns logic flow to the parent program.

FIG. 6 is a logic flow chart of the software in the fault logging subroutine 234.

Entering in block 502, block 504 enters the senor check subroutine 206 (see FIG. 5).

In block 506 the result of the sensor check performed in block 504 is evaluated. If it is determined that a sensor failure has occurred we proceed to block 534 and set the variable SYSTEM_SHUTDOWN=YES. Block 536 then performs any event logging specified by the system configuration (example: output to printer, output to message display). Block 538 energizes a remote dial module if the system is equipped with one.

Block 540 energizes the appropriated LEDs to indicate which system fault has occurred while de-energizing the LEDs that indicate positive system status.

Block 542 returns logic flow to the parent program.

If in block 506 it is determined that the sensors have responded properly, then block 508 evaluates the discharge side refrigerant temperature. If the refrigerant temperature is found to be within acceptable limits, block 510 is entered. Block 510 evaluates the liquid refrigerant pressure, and if the pressure is found to be within acceptable limits then block 512 is entered. Block 512 checks for refrigerant low pressure. If the refrigerant pressure is found to be above the low limit then block 514 is entered. Block 514 evaluates the calculated value for superheat to determine if it is within preset limits.

If the four system conditions evaluated in blocks 508, 510, 512 and 514 are all found to be within acceptable limits then the variable SYSTEM_SHUTDOWN=NO in block 532 is determined.

If in any of the four block 508, 510, 512 or 514 it is determined that the value is outside the acceptable range then a counter for that value is incremented by one (blocks 516, 518, 520 and 522). In blocks 524, 526, 528 and 530 each of the four fault counters are evaluated to see if any one of the four system values has faulted more than three times. If any one of the four system values has faulted more than three times the variable SYSTEM_SHUTDOWN=YES in block 534 is determined, otherwise the variable SYSTEM_SHUTDOWN=NO in block 532 is determined.

Logic then flows through blocks 536, 538, 540 and 542 as described previously.

FIG. 7 illustrates a display panel of the present invention.

This configuration consists of a main display and control module 602 and a separate diagnostic tool 624. The main display and control module 602 consists of a case 604 which houses the microprocessor 104 and other component parts. This case would typically be attached to the exterior of the refrigeration unit that it is protecting and controlling.

The LED display bank 160 can be seen on the face of the case 604. These LEDs indicate system conditions: “System OK” 606, “High Pressure Failure” 608, “Low Pressure Failure” 610, “Discharge Temperature Failure” 612, “Sensor Failure” 614, “Superheat OK” 616, “Superheat Warning” 618, “Superheat Failure” 620.

The hand held diagnostic tool 624 is connected to the main display and control module 602 through the use of a female modular connection 622 mounted on the case 604 and a male modular connection 628 connected to the hand held diagnostic tool 624 with an appropriate cable connection 630.

The hand held diagnostic tool 624 consists of a case 626 on which is mounted an LCD display 632 for quantitative viewing of system conditions monitored by the microprocessor 104. This diagnostic tool 626 is equipped with buttons 634 and 636 for selecting the system readings to be displayed on the LCD screen 632.

A manual reset button 638 is used by service personnel to reset the system after a lockout has occurred.

FIG. 8 is an illustration of an alternative device display panel with LED and LCD displays.

This configuration of the present invention consists of a main display and control module 702 without a separate diagnostic tool as in FIG. 7. The main display and control module 702 consists of a case 704 which houses the microprocessor 104 and other component parts. This case would typically be attached to the exterior of the refrigeration unit that it is protecting and controlling.

The LED display bank 160 can be seen on the face of the case 704. These LEDs indicate system conditions: “System OK” 706, “High Pressure Failure” 708, “Low Pressure Failure” 710, “Discharge Temperature Failure” 712, “Sensor Failure” 714, “Superheat OK” 716, “Superheat Warning” 718, “Superheat Failure” 720.

An LCD display 722 is mounted on the case 704 for quantitative viewing of system conditions monitored by the microprocessor 104. Buttons 724 and 726 are used to for select the system readings to be displayed on the LCD screen 722.

A manual reset button 728 is used by service personnel to reset the system after a lockout has occurred.

U.S. Pat. No. 6,318,108, granted Nov. 20, 2001, is incorporated herein by reference in its entirety for all useful purposes. As explained therein a water spray onto the heat exchange coil of a condenser is utilized to wash and clean the condenser coil for more efficient operation. 

1. A cooling system including a compressor, a condenser having a fan motor and a heat exchange coil, and an evaporator with refrigerant flowing through the system, an outside ambient temperature sensor for determining outside ambient temperature, and a device for constantly calculating sub-cooling of the refrigerant and determining when the sub-cooling is below a predetermined level at a given ambient temperature, and generating a signal representative of the determined sub-cooling.
 2. A cooling system as in claim 1 including cooling means for lowering the temperature of the refrigerant when the determined sub-cooling is below the predetermined level.
 3. A cooling system as in claim 2 wherein the cooling means is water spray onto the heat exchange coil of the condenser.
 4. A cooling system as in claim 2 wherein the cooling means is an air flow onto the heat exchange coil of the condenser.
 5. A cooling system as in claim 1 including a fan limiting device constructed and arranged to cycle the condenser fan motor at ambient temperatures below a predetermined level to thereby maintain a substantially constant pressure on the refrigerant flowing through the cooling system.
 6. A cooling system as in claim 5 wherein the predetermined temperature level is within the range of 60° to 70° F.
 7. A cooling system as in claim 5 including a device for determining pressure of the refrigerant flowing through the cooling system and for deenergizing the compressor when the refrigerant pressure drops below a predetermined level.
 8. A cooling system as in claim 7 wherein the device for deenergizing the compressor when the refrigerant drops below a predetermined level also deenergizes the compressor when the refrigerant pressure is above a predetermined level.
 9. A cooling system as in claim 8 including a display of refrigerant temperature and pressure.
 10. A cooling system as in claim 7 including a device for determining superheat of the refrigerant and for deenergizing the compressor when the superheat drops below a predetermined level.
 11. A cooling system as in claim 10 including display means for indicating a low superheat warning within a range slightly above the predetermined level below which the compressor is deenergized.
 12. A cooling system as in claim 11 including a display for indicating satisfactory superheat, superheat warning and superheat failure.
 13. A cooling system including a compressor, a condenser having a fan motor and an exvaporator with refrigerant flowing through the system, an outside ambient temperature sensor for determining outside ambient temperature, and a fan limiting device constructed and arranged to cycle the condenser fan motor at ambient temperatures below a predetermined temperature level to thereby maintain a substantially constant pressure on the refrigerant flowing through the cooling system.
 14. A cooling system as in claim 13 wherein the predetermined temperature level is within the range of 60° to 70° F.
 15. A cooling system as in claim 13 including a device to determine pressure of the refrigerant flowing through the cooling system and to deenergize the compressor when the refrigerant pressure drops below a predetermined level. 